This invention relates to secondary alkaline batteries and, more particularly, it relates to separator systems for use in such batteries.
As described, for example, in Falk and Salkind, Alkaline Storage Batteries, pp. 168-170 (1969), it is well known that it is advantageous to interpose a separator system between the electrodes of opposite polarity in rechargeable alkaline batteries, such as silver/zinc and nickel/zinc batteries. In general, such separator systems include materials which are permeable to the electrolyte, but which reduce the migration of ionic or molecular species from one electrode to the other and retard, it not inhibit, dendritic growth from one electrode toward another.
The separator system may include a single separator made up of one or more layers of a semi-permeable membrane made from, for example, cellophane, polyethylene or polypropylene. More commonly, a separator system will comprise (a) a main separator which is a semi-permeable membrane as described, together with (b) a spacer or "positive interseparator" forming a macro-porous barrier between the oxidizing (+) electrode and the main separator and (c) a spacer or "negative interseparator" positioned between the reducing (-) electrode and the main separator. The material forming the positive interseparator is usually an inert base polymer such as nylon, polypropylene, or a vinyl chloride/acrylonitrile copolymer (Dynel), whereas the negative interseparator may be formed from a cellulosic material, nylon or polypropylene felts or non-woven fabrics.
The negative interseparator serves, or at least is intended to serve, several functions in a cell. It imparts mechanical strength to an electrode, particularly when that electrode is composed largely of a powder, such as zinc oxide. Additionally, it maintains non-adherent insoluble oxides (which may be formed at the negative electrode during charge/discharge cycling) in direct contact with the negative electrode during the charging process so that the required electron transfer can occur. Another function of the negative interseparator is to maintain electrolytic contact across the face of the electrode, which it can do by acting as a wick. The negative interseparator should also inhibit dendritic growth from the negative electrode.
In batteries containing, for example, silver/zinc or nickel/zinc cells together with the described separator systems and materials, it has been found that dendrites of metallic zinc grow from the zinc electrode into, and eventually across, the separator material after numerous charge/discharge cycles. This dendritic growth causes the cells to short circuit. Additionally, it has been found that after cycling, the negative interseparator becomes plated with zinc metal and penetrated by zinc particles. In that condition, it is not capable of performing its intended function.
Although presently-available negative interseparators provide an advantage in alkaline rechargeable cells over their non-use, ion migration, dendritic growth, and current density non-formity still remain significant problems.